Naturally-occurring water supplies often contain dissolved minerals to varying degrees. Such minerals include dissolved salts of aluminum, iron, chromium, zinc, manganese, nickel and cobalt. It is customary to filter such water supplies, especially if they contain suspended solids, but filtration does not remove dissolved salts. When the water is later used in such processes as laundries, dishwashing, bottle-washing, etc. where alkaline detergents, builders and the like are employed, the metal ion may precipitate as the hydroxide and stain any fabrics, glassware, etc. which it contacts. Thus a need exists for a method of inhibiting precipitation of such metals under alkaline conditions.